Electric gaseous discharge device



May 10, 1938. FOULKE 2,116,678

' ELECTRIC GASEOUS DISCHARGE DEVICE Original Filed April 9, 1956 Fig. I

INVENTOR J ed E, Fouke I a" v ATTORNEY Patented Ma 10,1938

ELECTRIC GASEOUS DISCHARGE DEVICE -Ted E. Foulke, Nutley, N. J.,assignor to General Electric Vapor Lamp Company, Hobokcn, N. J.,

a corporation of New Jersey Original application April 9, 1936, SerialNo.

73,533. Divided and this application January 14, 1937. Serial No.120,543

3 Claims.

The present invention relates to electric lamps containing a novelfluorescent material as a light source.

A particular object of the invention is to provide a gaseousdischargedevice having an internal coating of a novel fluorescent material tocomplement or modify thelight emitted thereby. Another object of myinvention is to provide a light source of this type which will have ahigh efficiency throughout a long useful life. Still other objects andadvantages of the invention 1 will appear from the following detailedspecification. The invention consists in the novel gaseous discharge;device hereinafter set forth and claimed. In t-iie'artlficialpreparation of luminescent substances it has long been known that thebest results were obtained by bringing the main luminesc'entsubstanceinto intimate contact with a dsmall'amount of another material whichserves as an exciting agent, usually in the form of a solid 'solution,and that this result was materially aided through the use of a flux,such as sodium carbonate. While many well known luminescent mixtureshave been produced by this method, all prior efforts to produce aneffective luminescent materialcontaining cadmium silicate according tothis procedure have failed, the luminescence of the product thusproduced being so weak and inefficient that the product was valuelessfor any practical purpose. This has been true of all cases whereordinary so-called chemically pure cadmium oxide, silica, manganesedioxide and any of the usual sodium, barium, strontium or calcium fluxeshave been sintered together, in accordance with the common practice, toproduce a luminescent compound. As a result no hope has been entertainedof being able to use cadmium silicate as a source of luminescent light,in spite of the desirable red color of the light emitted thereby.

I have now discovered that this failure to produce a luminescent productof practical value has been due to the presence of minute quantities ofnickel, iron, cobalt, copper, lead or the like in the compounds used,and that these traces of foreign elimination'of the luminescence of theresulting product. Consequently the fundamental step of my novel methodis a complete elimination of all of these undesired metals from the.product, either by the initial purification of the materials employed,or by removal during the heating incident to the production of the finalproduct. The purification of the materials themselves is, of course, atedious and expensive'process, so that elements have been responsiblefor the virtual the latter method is to be preferred, especially since Ihave now discovered that the presence of potassium in the mixture whichis calcined to produce the luminous material results in the completeelimination of these undesired metals, or at least of their undesiredeffects. At the same time this potassium serves'as a flux which verymaterially speeds up the desired production of a solid solution of theactive fluorescent elements, with the result that with this novel methoda highly efficient luminescent product is formed within a few minutes oreven seconds, as contrasted with the hours or days which would otherwisebe required. It is thus apparent that I the potassium or potassiumcompounds employed have a dual function not possessed by any of thefluxes heretofore used, and that the potassium thus produces a resultwhich is unattainable with any of these other fluxes.

I have furthermore discovered that the ratio of the various elementsemployed is critical and that the desired result is not produced unlessthe potassium is kept within certain narrow limits. Thus I have foundthat first of all the ratio of the cadmium oxide to the silicic acidwhich is commonly employed should equal .811, since there is a rapiddecrease in the luminescent effect with variations on either side ofthis value. While this is substantially the molecular ratio required toproduce the cadmium silicate CdSizOs it is not certain that thiscompound is necessarily produced, since it is possible that a solidsolution of CdO.2SiO2 is formed. Secondly, the maximum luminescenteffect is produced when the manganese dioxide, silver, or other excitingagent and the potassium used together constitute a critical percentageof the total mixture. For example, where manganese dioxide andpotassium. chloride are used, the total percentage of these compoundsfor maximum'brightness of the luminescent material should beapproximately 3%, the luminescence obtainable falling off to less than ahalf when this percentage is decreased to 2% or increased to 4%, andpeaking sharply at 3.1%. Thirdly, the ratio of the potassium chloride tothe manganese dioxide is critical and should be approximately 2 for bestresults. These ratios and percentages, all by weight, are for the,particular compounds mentioned, it bcingunder stood that where othercompounds of manganese and potassium are employed these figures arevaried to keep the proportions of the metals themselves constant, thatis, for best results the potassium should be approximately 2.1% of thecadmium oxide, by weight, while the manganese is approximately 1.25% ofthe cadmium oxide.

The presence of potassiumin the resulting product has been found to bevery beneficial inthat it produces a crystalline substance which iseasily pulverized to produce any desired crystal size. such as desiredfor the coating of the inner surface of mercury vapor arc lamps andother similar devices. While this pulverizatlon results in a substantialloss in the luminescent effect, as is well known, I have discovered thatby heating the crystals to a temperature just below that at which theywill again sinter together their luminescent property is restored to themaximum value. Thus this novel process results in the production of apowder of any desired fineness and having highly efficient luminescentproperties. In passing it may be noted that this novel process ofrestoring the luminescent property to powdered material has been foundtobe effective with \some fifty inches long, has a cathode 2 at one allof these luminescent compounds, regardless of the flux used. y Y

The presence of the potassium in my novel luminescent material has alsobeen found to greatly enhance the stability of the product as comparedto that which is attainable in any other way, it having been observedthat acoating of this material used within a mercury vapor are lamp isunaffected either by the discharge or by the mercury vapor and hencedepreciates inluminous efliciency at an extremely low rate.

For the purpose of illustrating my invention I have shown a nove lamphaving my new coating on the inner surfa e thereof in the accompanyaddedto this mixture and stirred to form athin paste. after which manganesechloride is added and stirred, the proportions being such that thepotassium is substantially 2.1% of the cadmium oxide and the manganeseapproximately 1.25% of the cadmium. oxide, by weight. At this stage themanganese chloride and potassium carbonate interact to produce manganesecarbonate,.-which loses C02 upon heating) and potassium chloride, thisparticular method of introducing these com pounds having been found togive an exceptionally good distribution of these compounds throughoutthe mass. The mixture is then dried and stirred to further mix it, afterwhich it is heated in air to a temperature of the order of 1100 C. forapproximately -five minutes or until the fuming stops. During thisheating the potassium cloride unites with any traces of the oxides oflea nickel, iron, cobalt, copper or the like, all of which have beenfound to be undesirable, to form chlorides of these metals and potassiumoxide, all of which are volatilized at or below this temperature, thusaccounting for the fuming during this heating. At the same time theremaining major proportion of the potassium chloride fuses and serves asa flux which causes the cadmium, the silicon and the manganese compounds to rapidly go into the desired solid solution.

After cooling the resulting fritted mass is pulverized to the sizedesired and then again reheated for several minutes in the crucible to atemperature of the order of 1050" 0., this being just below thesintering point of the mass, and

then cooled, the resulting powder being ready for use and having anextremely high luminescent efficiency.

In some cases several intermediateheatings in the crucible may be addedto the above, each heating being at a slightly lower temperature thanthe one previous thereto, with pulverization following each heating,slightly improved results being obtained in some cases by these addedsteps, especially where the initial mixing of the various components ofthe product has not been entirely thorough.

The resulting powder is then affixed to the inner surface of the arctube l of the lamp shown in Figs. 1 and 2 As there shown this lamp,which is of the order of 1 inch in diameter and end thereof, anda pairof anodes 3 in separate chambers at the opposite end thereof inaccordance with conventional practice where rectifying operationonalternating current is desired; although it is to be understood thatby using a similar cathode 2 in place of the anodes-3 the tube couldalso be made to operate with a reversing discharge on alternatingcurrent. Likewise the device may be operated on direct current with asingle anode. the type disclosed in U. S. Patent 2,009,211, granted July23, 1935 to James A. St. Louis, but it is to be understood that any ofthe well known types of cold cathode. such as now'used 1in signs and thelike, or hot cathode suchas the Pirani or Hull cathodes, for example,are used-in place thereof where desired. The anodes are preferably ofgraphite, or any other suitable material. This lamp may contain anysuitable gaseous atmosphere, but I prefer to use argon and mercury,,,since a discharge in thisparticular mixture-pro-' duces slow electronsand radiations, suchsas the 2537 A. line, which are extremely effectivein exciting the luminescent cadmium silicate and likewise producesvisible radiations ,.which are. complementary to the red of thecadmium.- silicate, whereby the resultant light emitted by-the lampclosely approximates daylight. The argon is usually introduced at apressure of the order of 2 or 3 m. m. of mercury, while the mercury is-.

preferably restricted to an amount slightly in excess of that which is,vaporized at the, normal operating temperature, in order to preventexcess deposits of mercury along the tube in operation.

Where a mercury containing appendix is, em.-.;

ployed. however, more mercury is sometimes employed. The -luminescentcoating is aifixedto the inner surface of the tube l-,substant ially,allthe way from the cathode 2 to theanodes 3 either by means of a lowermelting point glass,...

as disclosed by Wilford J Winning-hoi'f in his pending application,Serial No- 511,409, filed Jan. 26, 1931, or by the use of a boric oxideglaze such as described by Eugene Lemmers et al. in their pendingapplication, Serial No. 741,144, filed August 23, 1934. or in any othersuitable manner.

When such a lamp is operated with a oneampere discharge with inductiveballast in a conventional rectifying circuit the arc drop isapproximately 88 volts and the total input I Serial Number 73,533, filedApril 9, 1936.

this efliciency is still further increased. I have found that my novelcoating, due to its unique character, is unaffected by the dischargeunder these conditions, so that the light output varies but little withage, the device thus having a useful life of many thousand hours.

While I have described my invention by reference to a process whereinthe potassium and manganese are added as separate compounds it is to beunderstood that in some cases they may be added in a single compound, asby the use of potassium permanganate. understood that other compounds ofpotassium and of manganese can be employed, and that various otherchanges, substitutions and omissions, within the scope of the appendedclaims may likewise be made by those skilled in the art in the steps ofthe method or in the product without departing from the spirit of myinvention.

This is a division of my co-pending application,

I claim as my invention:

1. An electric gaseous discharge device comprising a sealed envelopecontaining a gaseous atmosphere and having electrodes sealed therein, acoating of luminescent cadmium silicate on the inner wall of saidenvelope and extending along It is likewise to be.

the discharge path between said electrodes, said coating containing aflux of a potassium compound.

2. An electric gaseous discharge device comprising a sealed envelopecontaining mercury and having electrodes sealed therein, a coating ofluminescent cadmium silicate on the inner wall of said envelope andextending along the discharge path between said electrodes, said coatingcontaining a small percentage of a potassium. compound which serves as aflux.

3. An electric gaseous discharge device comprising asealed envelopecontaining mercury and having electrodes sealed therein, and a coatingof a luminescent material on the inner wall of said envelope andextending along the discharge path between said electrodes, said coatingconsisting of cadmium oxide and silicon dioxide in the proportionsindicated by the formula nese and potassium compounds which serve asctivator and flux, respectively, of the resulting luminescent material,the manganese and potassium, in terms of the metal content of thesecompounds, being of the order of 1.25% and 2.1%, respectively, of thecadmium oxide content of the luminescent material.

\jdsizOs having intermixed therewith manga- TED E. FOULKE.

